1. Field of the Invention
This invention is concerned with an improved catalyst and improved catalytic process for the demetalation and desulfurization of petroleum oils, preferably those residual fractions with undesirably high metals and/or sulfur contents. More particularly, the invention utilizes a demetalation-desulfurization catalyst characterized by a novel pore volume and pore size distribution. Additionally, this invention involves catalysts comprising a Group VIB metal and a Group VIII metal composited with an alumina support characterized by a content of gamma phase alumina and a specific pore size distribution.
2. Description of the Prior Art
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals and sulfur content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium. Iron and small amounts of copper are also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the premature aging of the catalyst and/or formation of inordinate amounts of coke, dry gas and hydrogen.
It is a current practice to upgrade certain residual fractions by a pyrolitic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical, and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor." This factor is equal to the sum of the metals concentrations, in parts per million of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25 or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker using for instance the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably about 90% need to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking chargestocks.
Metals and sulfur contaminants would present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1,000.degree. F. and a pressure of 100 to 3,500 psig.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur content of petroleum oils, and particularly of residual fractions of these oils. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail due to a very rapid deactivation of the catalyst, presumably by metal contaminants. Therefore, it is also evident there is considerable need for a demetalation/desulfurization catalyst possessing improved aging characteristics.
U.S. Pat. No. 3,770,617 describes a hydrodesulfurization process that employs a catalyst having an oxide or sulfide of a Group VIB and/or Group VIII metal on an alumina support characterized by a specific pore size range; U.S. Pat. No. 3,931,052 describes the demetalation and desulfurization of metal and sulfur containing residual petroleum oils through the use of a catalyst comprising a hydrogenating component composited on an alumina base, whose pores are substantially distributed over a narrow 180 to 300 A diameter range and U.S. Pat. No. 3,383,301 (Beuther et al.) which also deals with demetalation and desulfurization wherein an alumina base catalyst on which is composited a hydrogenating component is utilized.
U.S. Pat. No. 3,876,523 describes a demetalation/desulfurization catalyst capable of reducing the metals content (Ni+V) by as much as 88% and removing over 95% of the sulfur contaminants. However the performance of this catalyst upon aging leaves something to be desired.
It has now been discovered in accordance with the present invention, that a catalyst which has its pore volume substantially concentrated in certain narrowly defined pore sizes provides a catalyst of overall superior demetalation and desulfurization properties, i.e., such a catalyst provides high demetalation and desulfurization when fresh and when aged.
Some prior art catalysts such as described in U.S. Pat. Nos. 4,048,060 and 4,113,636 possess some properties similar to those of the instant catalyst but are devoid of others necessary for superior demetalation and desulfurization activity when fresh and after prolonged use. Especially significant are differences in the pore volume attributed to pores having diameters ranging from 0 to 100 .ANG..